This may sound like a pretty silly idea, but I use random wallpapers and it would be quite nice if the 'pool' from which a paper was selected could be changed by the time of day. Badly explained I know, but, for example, during the day a random choice could be made from certain categories only and, in the evening, the choice could be made from a different set. This would mean action scenes in the day and nice, quiet sunsets or seascapes in the evenings

You can assign time limits to each category you make (right click a category and go to "Wallpaper Categories" -> "Edit Category"). So to achieve what you're after, you would be making for example a "day" and a "night" category.

For the time limit of the "day"-category you put something like:
* from: 8.00
* to: 16.00